Orientation-Control of Two Plasmonically Coupled Nanoparticles in an Optical Trap

Hamideh Kermani and Alexander Rohrbach
Optical monitoring of nanoparticle (NP) dynamics is typically beyond the spatial and temporal resolution limit of light microscopy. However, the orientation and assembly of NPs can be controlled by various light scattering methods. Here we demonstrate how two 80 nm silver NPs form a dimer inside an optical trap and orient along the electric field of the trapping laser, therefore allowing to rotate them stably in the horizontal plane. We built a dual-path spectrometer for two orthogonal polarization directions to determine the azimuthal dimer angle for different plasmonic coupling strengths by the difference in the measured spectral intensity maxima. The azimuthal angle of the dimer could be retrieved with an accuracy of a few degrees independent of the spectral frequency or the distance between the NPs. Our results coincide well with a developed theoretical model predicting polarization-dependent scattering spectra for dimers with different orientations and NP distances. Our study points out another strategy for a highly controlled assembly of single NPs using optical tweezers and multimodal scattered light.

DOI